Robots
Robots is a simple micro-world modelled on the well-known "Karel the Robot" world.
The world holds robots, beepers and walls.
The Marine Biology Case Study, used in the US AP course, is a similar example, which can also easily be programmed in greenfoot.
In this example, the cell size in the world is much larger. The world consists of an 12x8 grid of cells.
Ants
A few anthills, some food sources, and lots of ants swarming out to find food.
Ants lay trails of pheromones if they find something, so that other ants can follow the trail.